Here is what I think about Dart: it is huge and is another programming planet that you can't reuse what you already have (like thousands of libraries in NPM). And the most important part; it looks boring. the articles about it look also are very boring.
Dart has a js-interop library for the times that you want to reuse some code written in JavaScript. This video gives a good introduction to it: http://www.youtube.com/watch?v=QFuCFUd2Zsw
And the most important part; it looks boring.
That's was a big design goal when they created the language. It's designed so that, for the most part, it looks and behaves like other languages that you're used to. You can pick it up in an hour or so and be productive right away. The Dart team didn't want to create some esoteric language with all sorts of crazy features, they set out to create a real language that people would feel comfortable using.